In recent years there has been a great deal of interest in preceramic polymeric materials which can serve as precursors, via their pyrolysis, for silicon-containing ceramics. R. W. Rice, Amer. Ceram. Soc. Bull., 62, 889-892 (1983). Use of such polymers include among others: formation into complex shapes and subsequent pyrolysis to give a ceramic material of the same shape; spinning into continuous fibers whose subsequent pyrolysis yields ceramic fibers; as a matrix material for carbon or ceramic fibers, or as a binder for ceramic powders (with subsequent pyrolysis to form a ceramic body); oxidation-resistant coatings on otherwise oxidizable materials (such as carbon-carbon composites), after the polymer coating is made it can be pyrolyzed to give the resistant ceramic coating; infiltration of porous ceramic bodies such as ones obtained from reaction-sintered silicon nitride by the polymer itself (if liquid) or by a solution of the polymer with subsequent pyrolysis to form a ceramic resulting in better strength, oxidation resistance, etc. of the body; and formation of thin films for electronics applications. For example, Penn, et al., J. App. Polymer Sci., 27, 3751-61 (1982) describe the preparation of silicon carbide-silicon nitride fibers from a polysilazane precursor. Other polymer precursors for forming silicon carbide and silicon nitride ceramics have been described in U.S. Pat. Nos. 3,108,985; 3,853,567; 3,892,583; 4,310,651; 4,312,970; 4,404,153 and 4,611,035. Seyferth, et al., in U.S. Pat. Nos. 4,650,837, 4,645,807, 4,639,501, 4,780,337, 4,767,876, 4,720,532, 4,705,532, 4,705,837, 4,719,273, 4,820,738 and 4,482,669 have disclosed new preceramic polymers whose pyrolysis results in good ceramic yields.
SiC-based ceramic fibers identified under the tradename Nicalon are currently commercially available and are prepared from preceramic polymers through a number of steps. Initially, a polydimethylsilane is prepared from the Na condensation of dimethyldichlorosilane. Upon heating to 450.degree. C. in an autoclave, the polysilane is converted to a polycarbosilane through a Kumada-type rearrangement. The polycarbosilane contains a backbone of alternating Si and C atoms and consists of both cyclic and linear units. This polycarboxilane is both soluble and fusable and can be spun into fibers. Further processing of the fibers is generally required, as they do not retain their shape upon pyrolysis. Such processing has included oxidative curing of the "green" fibers at 350.degree. C. before pyrolysis to give a mixture of SiC and SiO.sub.2. The introduction of oxygen in the cure step contributes to the formation of the silica, although the SiO.sub.2 can detract from the ceramic fiber's high temperature strength because of its lower crystallization temperature than SiC. In addition, high temperature reactions of SiO.sub.2 with C or SiC can lead to the production of CO and SiO gases. These gases can reduce the ceramic strength by forming void spaces in the ceramic.
West, et al., Am Ceram. Soc. Bull., 62, 899 (1983), reported synthesis of soluble polymethylsilane copolymers from the Wurtz coupling of MePhSiCl.sub.2 and Me.sub.2 SiCl.sub.2. Fibers produced from this "polysilastyrene", while not necessarily requiring an oxidative cure step, do generally require a UV irradiation cure to crosslink the fibers so that they retain their shape upon pyrolysis. The ceramic yield of the these fibers was reported to be only 15% at 1100.degree. C. Baney, et al., Organometallics, 2, 859 (1983), reported production of an alternative silicon carbide precursor by the redistribution of methylchlorosilanes. This polymerization reaction, catalyzed by tetrabutylphosphonium chloride, was reported to yield polycyclic polymers of the formula (Si.sub.x Me.sub.y Cl.sub.z).sub.n and methylchlorosilane monomers. The cyclic polymers are said to be converted into SiC with a 47% ceramic yield upon pyrolysis to 1200.degree. C. Schilling and Kanner in European Patent Application 123,934; and Chem. Abstr., 102:79465m, reported preparation of a polysilane from the Wurtz-coupling of Me.sub.3 SiCl with the unsaturated halosilane, ViMeSiCl.sub.2 in 36% yield. This polysilane, containing primarily Me.sub.3 Si and MeSiVi groups, when pyrolyzed to 1200.degree. C. was reported to give SiC in 38.5% ceramic yield.
More recently, Harrod, Laine, and others in J. Am Ceram. Soc., 74, 670 (1991), reported that a polymethylsilane polymer can be synthesized by dehydrogenative coupling of methylsilane, CH.sub.3 SiH.sub.3, with Cp.sub.2 TiMe.sub.2. The soluble polymethylsilane was reported to provide a ceramic yield of 77% at 1000.degree. C. with an average elemental analysis of Si.sub.1 C.sub.0.9 H.sub.&lt;0.2 O.sub.0.1. Large scale use of methylsilane may be undesirable, however, as the reagent is costly and a potentially dangerous gas, forming explosive mixtures with air.
The catalytic dehydrogenative polymerization of Si--H moieties by some group 4 metallocenes has been reported. See, for example, Aitken, et al., J. Organomet. Chem., 279, C11 (1985); Harrod, Inorganic and Organometallic Polymers, ACS Symposium Series 360, ch. 7, Zeldin, et al., ed., (1988); Aitken, et al., Organometallics, 8, 1732 (1989); Laine, Aspects of Homogeneous Catalysis, vol. 7, p. 37-63, Ugo, R., ed., Kluwer Academic Publishers, Netherlands (1990). Generally, however, such reactions have worked well only with primary silanes. Relatively poor results have been reported for secondary Si--H groups, and tertiary silanes have been reported to be generally unreactive to such catalytic polymerization. As used herein, the term secondary Si--H group or secondary silane refers to as Si group that is bonded to only two hydrogen atoms and the term tertiary Si--H group or tertiary silane refers to a Si group that is bonded to only one hydrogen atom.
A polysilane has been reported, by T. G. Wood, Ph.D. disseration, Massachusetts Institute of Technology (1984), and in U.S. Pat. Nos. 4,537,942, 4,611,035, and 4,704,444. It has been found, however, that when this polysilane is pyrolyzed ceramic residues are provided in somewhat modest yields and the resulting ceramic can contain significant amounts of free silicon. As used herein, a polysilane is an organosilicon polymer whose backbone contains Si--Si bonds.
Excessive amounts of free silicon or free carbon is undesirable for a ceramic product. For example, elemental silicon has a melting point of 1410.degree. C. Thus, the presence of a significant amount of free silicon limits high temperature applicability of a ceramic.
It would be desirable to have a preceramic polymer whose pyrolysis can provide a significantly higher yield of ceramic that contains little free silicon and/or little free carbon.